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sammy
11-26-2006, 10:56 AM
Hello all, nubie here. Just put together my 20" A4. All Rock River componets except for a Jewell trigger. I cleaned it throughly and lubed with CLP using the Bushmaster guide online. Now that it is all together I took it out today ind this is what is happening.

First 10 rounds fed flawlessly but the bolt would not stay locked back after the last round.

The next magazine, first round fired and ejected but did not chamber the next round. This happened every round untill it was empty, and it failed to lock the bolt on this magazine either.

Using a different magazine I tried again. Fired and fed 5 out of 10 rounds, and did lock back the bolt this time.

Fourth try on yet a different magazine and different ammo 4 out of 10 failed to feed and this time the bolt locked back.

This happened using Guat surplus and Remington UMC. 3 New Bushmaster magazines, I am at a loss here. I just spent 2 hours cleaning and lubing and cannot see any obvious problems.

Thanks for looking, Sammy

Joe
11-26-2006, 11:28 AM
For the bolt not locking back:

It definately sounds like the bolt catch. I had the exact same problem with my rifle. The bolt catch itself had dings lumps on it that prevented it from moving freely in the lower. Make sure you used the correct spring:
http://www.calguns.net/calgunforum/showthread.php?t=32797&highlight=thread

PLINK
11-26-2006, 12:39 PM
I am thinking bolt catch also. I had a problem where the the bolt catch plunger was not moving freely in the hole. I need to clean the hole up with a drill bit to get it to move like it should.

sammy
11-26-2006, 1:41 PM
I checked the bolt catch and it looks fine. The empty magazine moves the catch to the up position. When the magazine is removed it moves to the down position. The bolt catch locks the bolt when cycling the charging handle by hand. I posted this on ar15.com and they thaught the problem might be with the 10 round bushmaster magazines. Again it does not chamber a new round but ejects the empty shell every time. Thanks, Sammy

Joe
11-26-2006, 2:00 PM
I have 3 new bushmaster mags and have had NO problems with them.

DULLYJAY
11-26-2006, 2:11 PM
Take a look at bolt carrier. Gas key might be loose(I think that is what its called.).

sammy
11-26-2006, 2:31 PM
Technical Ted I checked to see if the nose of the round is dragging and it is possible. The length of the guat. surplus and the UMC is the same but fit for the rounds is very tight. It is possible that these magazines were built at the same time (they were a single purchace). I will get another and see what happens. Thanks, Sammy

Tzvia
11-26-2006, 3:08 PM
I had that problem on one rifle. The gas seal from the gasblock to the barrel was not so good. You could see the soot from the gas very easily on the stainless barrel. Replacing the gasblock solved the issue.

It sounds like it is shortstroking, pulling back just enough to eject, but not far enough back to catch the next round and push it forward or stay open on the last round. Aside from the key, make sure the pin that holds the gas tube in the gasblock is there. Also, if the gasblock is not the standard FS with the drilled through pins, verify the hole in the block is over the hole in the barrel and not obstructing it (too far forward or aft for example).

Also make sure if using a carbine buffer tube, that you are using a carbine spring and buffer.

50BMGBOB
11-26-2006, 8:31 PM
+1 on short stroking. If it doesn't feed each time that it is fired but will if you pull the charging hammer. I had to enlarge the gas hole in my bolt carrier to line up with the hole in the key (rare).

snacks
11-26-2006, 8:35 PM
i would expect the mags as well. I had a bushmaster mag that had difficulties feeding. After using it a few times and a bit of lube it shot fine. Hopefully the problem resolves itself once the entire gun is broken in.

Rivet
11-26-2006, 8:36 PM
+2 My guat was short stroking also, try some other ammo.